OBJECTIVE To analyze longitudinal changes in the retinal nerve fiber layer (RNFL) thickness in branch retinal vein occlusion (BRVO) using optical coherence tomography. METHODS The authors prospectively analyzed 20 eyes in 20 patients diagnosed with branch retinal vein occlusion (BRVO) and followed for more than 1 year. The RNFL thickness of the normal and occluded eyes was measured at the time of diagnosis and at 1, 3, 6, and 12 months. The authors compared the changes in the occluded eye over the follow-up and the differences between two eyes at each time. They also analyzed the area opposite the occluded area. RESULTS The thickness of the RNFL in BRVO was significantly reduced at 1, 3, 6, and 12 months (P = 0.005 at 1 month and P = 0.001 at 3, 6, and 12 months) compared with initial thickness. The RNFL thickness at 3 months did not differ significantly between 2 eyes, whereas at 6 months and 12 months, significant (P = 0.032, P = 0.002, respectively.) thinning was observed in the occluded eye. Analysis of the area opposite that of BRVO revealed no significant (P > 0.05) change during the follow-up and no difference between the 2 eyes. CONCLUSIONS There was a significant decrease in RNFL thickness over time in BRVO and significant thinning at 6 months compared with the normal eye. Retinal nerve fiber layer thinning needs to be differentiated from glaucoma or systemic disease, but it should be considered the natural course after BRVO.